Apply tag only if env variable exists drone.io

I am using drone CI with the docker plugin and I would like to create an image tag using the commit, the branch name and the tag if any of those exist in the event that triggered the pipeline. How would I do that?

I've tried using defaults to avoid the build crashing because "foo/bar:" isn't a valid tag but this is not an ideal solution and for the branch name I need to replace a part of the string and I cant find a way to do both.

So, how would you apply a tag only if the env variable exists?

You can use the when to make sure the env variable is only used when it exists:

    - name: docker_build_tag
      image: plugins/docker
      when:
          event: tag
      settings:
          username:
              from_secret: docker_hub_username
          password:
              from_secret: docker_hub_token
          tags: ${DRONE_TAG}